This invention relates to a rubber composition excellent in processabilities such as rolling processability and extrusion processability before vulcanization and excellent in weather resistance and heat resistance and high in hardness after vulcanization.
Ethylene..alpha.-olefin rubbers and isobutylene-isoprene rubber (abbriviated to "IIR" hereinafter) have been widely used because of their excellent heat resistance and weather resistance.
However, when a high hardness rubber of at least 85 in hardness (as measured by the JIS-K6301 Hardness Tester Type A) is made from these rubbers there are practically many problems due to their very poor productivity and processabilities such as rolling processability and extrusion processability.
Hitherto, for production of such high hardness rubbers excellent in heat resistance and weather resistance there have been known, for example, a method according to which to ethylene..alpha.-olefin rubber and/or IIR excellent in heat resistance and weather resistance are added reinforcing agents such as carbon black and inorganic fillers in a large amount and a softener and a plasticizer in a small amount and a method where a thermosetting alkylphenol resin is added to the rubbers.
However, the former method has problems in processing, namely, Mooney viscosity of the unvulcanized rubber mixture (referred to as "compound" hereinafter) raises sharply, the mixing performance of Banbury mixer and rolling processability of the compound are poor, and furthermore feedability of the compound to an extruder is inferior and even if it could be fed to the extruder the extrusion speed is very low and load of the extruder increases too much resulting in breakage.
According to the latter method the rolling processability and the extrusion processability can be improved as compared with the former method. However, this method still has the problem that since the curing reaction of the alkylphenol resin is a dehydration condensation reaction, water is produced with the curing reaction upon elevation of temperature to vulcanization temperature and this water is released in a gaseous form and acts as a foaming agent to cause incorporation of bubbles in vulcanized rubber when an atmospheric continuous vulcanization is employed.
This dehydration condensation reaction has high reaction speed and produces water instantaneously. Therefore, this water cannot be completely captured with a dehydrating agent such as calcium oxide.
The above method can be used when vulcanization is effected under pressures such as steam vulcanization or press vulcanization, but such vulcanization is inferior in productivity to the atmospheric continuous vulcanization.